1. Field of the Invention:
This invention relates generally to waveguide structures such as simple waveguides, waveguide filters, and the like. The invention relates more particularly to a novel method of fabricating such structures.
Prior Art:
As will appear from the ensuing description, the fabrication method of the invention may be utilized to fabricate a variety of waveguide structures. However, the method is particularly suited to fabricating lightweight, high precision waveguide structures having a high degree of dimensional stability in a thermal environment whose temperature fluctuates over a relatively wide range, that is extremely small, if not virtually zero, dimensional change in response to the temperature fluctuations. Accordingly, the invention will be described in this particular context.
One example of a waveguide structure which is required to be fabricated with great precision and to possess a high degree of dimensional stability is that described in U.S. Pat. No. 3,697,898. This patented waveguide structure is a plural cavity bandpass microwave filter having a tubular body and reflection iris discs within the filter passage at positions spaced there along. These iris discs form therebetween resonant cavities which are coupled to one another and to the waveguides leading to and from the filter through iris openings in the discs. Threaded in the filter body between the iris discs are tuning screws which project into the resonant cavities and are adjustable into and from the cavities to tune the filter. Coupling flanges are provided at the ends of the filter body for joining the filter to the waveguides leading to and from the filter.
The operating principle of this filter is fully explained in the patent and hence need not be elaborated on in this disclosure. Suffice it to say that proper operation of the filter requires precise dimensioning of the filter cavities and high dimensional stability of the filter structure such that the cavity dimensions remain unchanged even though the filter structure is exposed to widely fluctuating temperatures. Such dimensional stability, in turn, requires fabrication of the filter body from a material having a very low and preferably virtually zero coefficient of thermal expansion. Light weight is another critical requirement of many waveguide structures.
A variety of materials exist which possess a sufficiently low coefficient of thermal expansion to satisfy the stringent thermal stability requirements of the patented plural cavity bandpass waveguide filter and other high precision waveguide structures. Examples of such low thermal expansion materials are various INVAR and KOVAR compositions such as Fe-35Ni, 54Fe-28NI-18Co, and 37Fe-30NI-25Co-8Cr; cermet and ceramics/metal composites such as Ni-60Al.sub.2 O.sub.3 and Al-20Al.sub.2 O.sub.3 ; ceramic or metal filler reinforced plastic materials wherein the filler components may be chopped fibers, whiskers, or powders of carbon/graphite, Al.sub.2 O.sub.3, ZrO.sub.2, fused silica, or INVAR, and the matrix may be epoxy, phenolic or other polymeric materials functioning as a binder.
While such low thermal expansion materials exist, each presents problems in connection with waveguide fabrication. From the standpoint of its thermal expansion and mechanical strength, for example, INVAR is ideal for a waveguide body. INVAR, however, is difficult or impossible to fabricate into a lightweight waveguide structure by conventional techniques, such as machining, electroforming, and the like. Other problems which are confronted in waveguide fabrication are application of a conductive liner to the interior surface of the body and, in the case of a waveguide filter, installation of irises within the waveguide filter body. In this regard, attention is direction to U.S. Pat. Nos.: 2,826,524 3,247,579 3,536,800 2,870,524 3,299,492 3,540,119 3,070,873 3,314,130 3,548,345 3,195,079 3,372,471
which illustrate various waveguide structures and methods of waveguide fabrication. This invention obviates these and other problems associated with fabrication of waveguide structures from materials of the class described.